Casting is probably the most important traditional moulding method. A melting charge of the material to be processed is cast into a mould in which it then solidifies to produce the casting.
Frequently, so-called lost moulds are used. Such moulds are produced from moulding sand, i.e. quartz sand and a bonding agent. Such moulds are usually formed by taking a mould from models. The liquid material is then cast into the mould. After the material has hardened, the moulding sand can be removed, i.e. the casting is unmoulded, whereupon the mould is destroyed. For this reason, this type of mould is known as a lost mould.
In order to optimize mould production the sand must, inter alia, be supplemented with suitable bonding agents. When producing moulds, it is thus essential to ensure that the properties of the moulding sand to be used are as best suited to the material as is possible. Thus, for example, the casting material used and the associated melting charge temperature and also the external and if appropriate the internal contour of the mould must be taken into account.
The quality of the moulding sand primarily depends on the clay content, the grain size and distribution, the shape and surface area of the quartz bodies, the type and quantity of the auxiliary materials, the moisture content and the degree of compression.
For economic and environmental reasons, the spent moulding sand is usually processed and reclaimed as completely as possible; the more so because 5 to 15 parts by weight of moulding sand is generally used per unit weight of casting. At least 90% of the spent moulding sand can be processed and fed back into the moulding circuit, since the bond coatings are usually still effective, so that only water and occasionally bonding agent has to be added. A portion of the old sand is discarded from the circuit and replaced with fresh replacement material.
Frequently, clay bonded moulding sands are used, which are usually fed back into the preparation method following the casting process, where appropriate quantities of water, bonding agent (for example bentonite), additives (for example coal dust) and fresh sand are again added to the old sand.
Processing is generally carried out in a mixer and usually under vacuum, in order to cool the moulding sand down at the same time. On processing, care has to be taken to ensure that the bonding agent coats the quartz sand grains in an optimal manner.
The intention of this processing is so that the processed sand leaving the mixer is of uniform quality. The quality of old foundry sand, however, varies due to the thermal load during the casting process which depends on the production program used, so that constantly, old sand with a variable moisture and clay content is fed back to the processing plant.
The perpetual aim of a properly functioning process is thus to detect variations in the old sand and to correct them by taking corrective measures in the preparation process, such as adapting the water addition or the bond content.